1. Field of the Invention
The present invention relates to a wavelength tunable multiplexing and demultiplexing filter device which is used for adding and dropping wavelength components (add-drop) in an optical network system to demultiplex and multiplex light of arbitrary wavelengths from optical signals, and to a wavelength routing device which uses the wavelength tunable multiplexing and demultiplexing filter device.
2. Discussion of the Related Art
With the progress of technologies used for optical wavelength multiplexing division and transmission in optical communication systems, a more versatile and more flexible optical transmission system has been required. In an optical transmission system, light of a desired wavelength must be picked up from an optical signal where wavelengths have been multiplexed at an arbitrary node, or light of a desired wavelength must be added to such optical signals to send them as wavelength domain multiplexed light.
FIG. 1 illustrates the configuration of a conventional four-channel optical add-drop system. In this figure, wavelength-multiplexed light is added to an optical demultiplexing filter device 101 and then the desired wavelengths, for example, λ1, λ2, λ3, and λ4 are demultiplexed from the multiplexed wavelengths λ1 through λn. Light of the demultiplexed wavelengths goes to optical switches 102a through 102d. Signals of newly modulated wavelengths λ1 through λ4 simultaneously enter the optical switches 102a through 102d, respectively. The optical switches 102a through 102d have two input terminals and two output terminals. They select the signals demultiplexed by the optical demultiplexing filter device 101 and the newly modulated signals of λ1 through λ4 based on external control signals, and then output them from the two output terminals. The output from each of the optical switches is given to a multiplexing filter device 105 via optical tunable attenuators 103a through 103d and splitting devices 104a through 104d. The splitting devices 104a through 104d extract some of the output from the optical tunable attenuators 103a through 103d and then output it to a monitor 106. The intensities of wavelengths are adjusted at the same level by controlling the output from the optical tunable attenuators via the monitor 106.
For such a conventional optical demultiplexing filter device, the selected wavelengths are previously fixed at λ1 through λ4. The number of channels will increase to dozens or several hundred in the near future. This will require multiplexing or demultiplexing light on that number of channels when arbitrary wavelengths are to be selected. For this reason, when an arbitrary channel is selected, all the channels must be temporarily disassembled and then the necessary channel must be selected from them. This causes disadvantages in making the scale of the communication system larger and increasing the transmission losses. At ordinary transmission nodes other than those using optical cross connections in a large-scaled matrix, the ability to select optical signals on several channels is good enough for the purposes of routing, add-drop, and it is desired that arbitrary wavelengths can be selected from a number of channels unlike the conventional fixed wavelengths.